As microelectronics technology continues to refine integrated circuit structures of reduced minimum feature size, high integration density and signal processing speed, the flexibility of semiconductor wafer structure parameters has been reduced to the point where nearly all components have become constituent and dimensioned-critical. As an example, in semiconductor environments where signal processing speed and robustness against environmental threats are important performance criteria, such as in very high speed integrated circuits, electroplated gold, which possesses superior conductivity and external influence immunity properties, has become the predominate choice at the interconnect medium.
The use of gold electroplate, however, is not without a price, as gold does not readily adhere to semiconductor (e.g. silicon) wafer structures and, consequently, requires the use of an adhesion layer such as titanium between the gold and the underlying semiconductor material. As a further complication, when deposited directly upon a titanium layer, gold forms compounds that cannot be readily etched.
One approach for remedying this problem of formation of detrimental metal compounds has been the addition of a diffusion barrier of titanium nitride between the gold electroplate and the underlying titanium layer, with an additional barrier layer of platinum between the titanium nitride and the gold, as described in the U.S. patent to Fournier U.S. Pat. No. 3,879,746. In the environment to which the patented scheme is applied, the interconnect methodology involves beam lead structures in which the contact surface area of the interconnect metal and the photoresist used for its patterning is considerably larger and, therefore, less prone to delamination during plating, than the extremely narrow (submicron) line widths of present day integrated circuit structures.
Substitution of the platinum layer by a gold layer reduces the photoresist adhesion problem but introduces a loss of adhesion between the titanium nitride and the gold layers. It has been found that the titanium nitride layer, as employed in the patented laminate structure, does not possess the necessary adhesion strength to prevent a very narrow width noble metal layer from separating from the nitride layer when subjected to successive temperature cycles. While the adhesion mechanism is not completely understood, it is believed that the fact that the electroplated gold is tensile while the titanium nitride is compressive, together with the extremely narrow (quasi edge) contact area of the noble metal and the nitride, contributes to the tendency of the gold line to separate or peel away from the titanium nitride when subjected to repeated temperature cycling.
In the course of our investigation of the problems of laminate adhesion and detrimental metal compound formation (silicon and gold have a 370.degree. C. eutectic temperature with a favorable free energy of formation and are therefore impacted by the 450.degree. C. temperature at which radiation damage anneals are routinely performed), we have confirmed that a reasonable thickness of stoichiometric titanium mononitride between the titanium and the overlying gold will prevent gold from diffusing into the underlying silicon areas when the structure is subjected to the above mentioned anneal for periods up to 30 minutes. In this investigation, no platinum layer as proposed in the above-referenced patent to Fournier was included.
However, adhesion of the gold to the titanium nitride remains a significant problem.
Our investigation has revealed that the addition of a thin (.ltoreq.500 .ANG.) layer of titanium between the titanium nitride and the overlying gold will improve the adhesion to over twice minimum standards required; however, the same titanium nitride barrier that had been previously impervious to gold diffusion now exhibits scattered failure sites. This effective porosity of the titanium nitride to gold can be eliminated by increasing the thickness of the titanium nitride by 40%; unfortunately, the difficulty in etching the resulting compounds formed between the gold and the titanium increases.